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Solifluction Processes in an Area of Seasonal Ground Freezing

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Solifluction Processes in an Area of Seasonal Ground Freezing PERMAFROST AND PERIGLACIAL PROCESSES Permafrost and Periglac. Process. 19: 31–47 (2008) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/ppp.609 Solifluction Processes in an Area of Seasonal Ground Freezing, Dovrefjell, Norway Charles Harris ,1* Martina Kern-Luetschg ,1 Fraser Smith 2 and Ketil Isaksen 3 1 School of Earth Ocean and Planetary Sciences, Cardiff University, Cardiff, UK 2 School of Engineering, University of Dundee, Dundee, UK 3 Norwegian Meteorological Institute, Blindern, Oslo, Norway ABSTRACT Continuous monitoring of soil temperatures, frost heave, thaw consolidation, pore water pressures and downslope soil movements are reported from a turf-banked solifluction lobe at Steinhøi, Dovrefjell, Norway from August 2002 to August 2006. Mean annual air temperatures over the monitored period were slightly below 08C, but mean annual ground surface temperatures were around 28C warmer, due to the insulating effects of snow cover. Seasonal frost penetration was highly dependent on snow thickness, and at the monitoring location varied from 30–38 cm over the four years. The shallow annual frost penetration suggests that the site may be close to the limit of active solifluction in this area. Surface solifluction rates over the period 2002–06 ranged from 0.5 cm yrÀ1 at the rear of the lobe tread to 1.6 cm yrÀ1 just behind the lobe front, with corresponding soil transport rates of 6 cm3 cmÀ1 yrÀ1 and 46 cm3 cmÀ1 yrÀ1. Pore water pressure measurements indicated seepage of snowmelt beneath seasonally frozen soil in spring with artesian pressures beneath the confining frozen layer. Soil thawing was associated with surface settlement and downslope soil displacements, but following clearance of the frozen ground, later soil surface settlement was accompanied by retrograde movement. Summer rainfall events caused brief increases in pore pressure, but no further soil movement. Surface displacements exceeded maximum potential frost creep values and it is concluded that gelifluction was an important component of slow near-surface mass movements at this site. Temporal and spatial variations in solifluction rates across the area are likely to be considerable and strongly influenced by snow distribution. Copyright # 2008 John Wiley & Sons, Ltd. KEY WORDS: solifluction; frost heave; thaw settlement; seasonally frozen ground; pore pressures INTRODUCTION of soil temperatures, frost heave, thaw consolidation, pore water pressures and downslope soil movement In this paper we report field measurements of solifluc- has allowed us to analyse the mass movement tion at Steinhøi, Dovrefjell, Norway (latitude 628 150 processes associated with soil freezing and thawing. 2800 N, longitude 098 510 5800 E, altitude 1396 m, An instrument package was developed for this Figure 1), where seasonal ground freezing and purpose during full-scale laboratory simulation thawing are from the surface downwards and where studies (Harris et al., 1996), and an equivalent permafrost is absent. Detailed continuous monitoring weather-proofed version was installed on a prominent solifluction lobe at Steinhøi in August 2001 (see Harris et al., 2007, for details). A lightning strike in * Correspondence to: Charles Harris, School of Earth Ocean and Planetary Sciences, Cardiff University, Cardiff CF10 3YE, November 2001 caused instrument failure which UK. E-mail: [email protected] was not repaired until 11 July 2002, and therefore data Received 17 September 2007 Revised 18 December 2007 Copyright # 2008 John Wiley & Sons, Ltd. Accepted 18 December 2007 32 C. Harris et al. Figure 1 Location of the Steinhøi solifluction monitoring station. only from the four year period from summer 2002 to The role of high soil moisture content in promoting summer 2006 are presented. The study is focused on gelifluction has been emphasised in many field studies processes rather than regional soil displacement rates. (e.g. Benedict, 1970; Washburn, 1979; Harris, 1981; Given the range of surface gradients, snow conditions, Jaesche et al., 2003; Kinnard and Lewkowicz, 2005) drainage, etc., within the Steinhøi area, coupled with and several authors have reported moisture contents annual climatic variability, it is likely that rates of close to the Liquid Limit (e.g. Washburn, 1967; Smith, solifluction are spatially and temporally highly 1988; Kinnard and Lewkowicz, 2005). During variable (see for instance, Washburn, 1999). laboratory simulations Harris et al. (1995, 1997, Although Andersson (1906) did not exclude non- 2003, 2008) demonstrated very low shear strengths periglacial environments in his original definition, the that persisted for only short periods immediately term solifluction has been widely used to describe behind the thaw front when void ratios were high. slow periglacial mass movement caused by frost creep Subsequent thaw consolidation rapidly lowered and gelifluction (e.g. Ballantyne and Harris, 1994; both void ratio and moisture content, and led to a French, 2007; Matsuoka, 1998, 2001). Gelifluction rapid increase in soil strength. The mobilised and frost creep apparently operate in tandem, since frictional strength of these thawing ice-rich soils both result directly from frost heave and thaw was largely a function of pore water pressure. High settlement. Where freezing and thawing are shallow pore pressures and low effective stress values were and of short duration, creep movements dominate (e.g. recorded in the laboratory experiments for short Matsuoka, 2005), while gelifluction occurs during periods (often only a few hours) immediately behind seasonal thawing of ice-rich soil (Washburn, 1967; the thaw front. Harris and Davies, 2000; Harris, 2007; Matsuoka, Continuous monitoring in the Alps allowed Jaesche 2001). Recent laboratory studies have shown that et al. (2003) to observe soil movements that lasted gelifluction is best considered as elasto-plastic several days at given depths in a thawing soil, while deformation that might be described as accelerated Kinnard and Lewkowicz (2005) in southwest Yukon pre-failure creep rather than viscous flow (Harris observed repeated discrete periods of displacement et al., 2003). rather than slow incremental strains. Similarly, Copyright # 2008 John Wiley & Sons, Ltd. Permafrost and Periglac. Process., 19: 31–47 (2008) DOI: 10.1002/ppp Solifluction Processes, Dovrefjell, Norway 33 Matsumoto and Ishikawa (2002) demonstrated that, on mantled by Weichselian till with clasts up to boulder a gelifluction lobe in Sweden, soil movement was size set in a frost-susceptible matrix of silt and concentrated in two periods when ice-rich parts of the soil fine sand. Solifluction processes have reworked profile, firstly near the surface and secondly at a depth of the till, and excavation during installation of instru- around 20 cm, were in the process of thawing. mentation some 15 m upslope from the lobe front The aim of the present paper is to describe con- revealed 1–1.2 m of reworked till overlying bedrock tinuous field data collected from a slope where and containing thin organic layers that likely extend solifluction processes are active, in order that the from the lobe front and represent palaeo-surfaces influence of soil thermal conditions, frost heave/thaw buried by frontal advance (e.g. Matthews et al., 1986, settlement and hydraulic conditions may be explored. 2005). Soil grain size distribution was measured from five samples collected at 10 cm depth intervals SITE DESCRIPTION to a maximum depth of 60 cm. Clay contents were less than 4% except for the sample from The site lies above the treeline on the southern flanks 50–60 cm which showed 13%. Silt contents rang- of Steinhøi (Figure 1). It consists of a well-developed ed from 16% to 38%. Occasional large clasts were not turf-banked solifluction lobe with a tread length of included in grain-size determinations. The soil has low approximately 30 m and riser height around 1.5 m. plasticity (Plasticity Index 5%) with Plastic Limit 27% (Figure 2). Turf-banked lobes are widespread across and Liquid Limit 32%. this slope. Bedrock comprises grey Proterozoic and The nearest meteorological station at Fokstugu Cambrian phyllite, biotite-phyllite and shale and (972 m), on Dovrefjell, has a mean annual air forms part of the early Ordovician Gula-nappe temperature of À0.18C (1961–90) and mean annual (Geological Survey of Norway, 2002). The area is precipitation of 435 mm. Hourly air temperatures Figure 2 The Steinhøi monitored solifluction lobe showing the frost heave frame, with the LVDT fixed base triangle in the foreground and logger box and solar panel in the background. The apex of the LVDT triangle is attached to a footplate embedded in the ground surface (see Harris et al., 2007, for details). Thermistors measuring air temperatures are shielded with white plastic tubing and attached to the solar panel pole. The station is fenced against large animals. Copyright # 2008 John Wiley & Sons, Ltd. Permafrost and Periglac. Process., 19: 31–47 (2008) DOI: 10.1002/ppp 34 C. Harris et al. measured 2 m above the ground surface at Steinhøi of 80 cm (Figure 3). Thermistors, shielded by 2 cm during 2003, 2004 and 2005 show annual means of diameter white plastic tubing, were also mounted on À0.18C, À0.88C and À0.18C, but mean ground the pole supporting the solar panel at heights of 50, surface temperatures were more than 28C higher, 100 and 200 cm. Temperature measurement errors are due largely to the insulating effect of winter snow. estimated to be up to Æ0.48C. The 10 cm thermistor Storms are common in winter, accompanied by strong proved unreliable, and data from this depth were not winds, and redistribution of snow by deflation leads to used. considerable spatial variation in snow thickness. Pore Water Pressure INSTRUMENTATION A vertical array of five Druck PDCR81 pore pressure transducers was installed at 10 cm depth intervals Instrumentation was designed to monitor three key between 20 and 60 cm. Transducers were filled with elements: ground surface movements (frost heave, low viscosity silicon oil and had a range of 350 mb, thaw settlement and downslope solifluction), ground combined non-linearity and hysteresis of Æ0.2% and and air temperatures, and pore water pressures.
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